Why Tomorrow's Forecast Changes Before Morning
Tomorrow is close enough to plan around and far enough that the details can still move. A high you saw at dinner can be a few degrees different at breakfast. The rain chance can jump, or a breezy afternoon can turn lighter. That movement is the forecast taking in newer observations, not a sign that the first number was made up.
Overnight is when a lot of those observations arrive. Surface stations, aircraft, satellites, and balloon soundings fill gaps that the evening map only estimated. If a front is faster, slower, wetter, or drier than the previous run expected, tomorrow's column changes. The useful habit is to read tomorrow once in the evening for the shape of the day, then again in the morning if your plans depend on it.
What usually moves
Rain chance moves more than temperature on many days, because showers depend on small features: a pocket of moisture, a sea breeze, a line of storms upstream. Shift that feature by a short distance and one town changes categories while the next town does not. Temperature moves when cloud cover changes. A morning that was expected to be sunny and is now overcast can take several degrees off the high. Wind shifts when the pressure pattern is refined.
The condition word can change even when the numbers barely do. "Partly cloudy" and "mostly clear" are neighbors. A switch between them is a small edit. A switch from "clear" to "rain" is a large one and deserves a look at the morning, afternoon, and evening blocks, not only the daily high.
How to keep the part you need
Write down the decision, not every digit. If you need to know whether to leave a window open, the question is whether rain is likely in your hours. If the chance stays under your cutoff after the morning update, you are done. If you need a layer for an early start, compare tomorrow's low with what you wore today. A two-degree wobble does not change the layer. An eight-degree wobble might.
There is rarely a benefit to refreshing the page every hour overnight. Sleep, then read the updated high, low, rain chance, and wind. The update time on the page tells you whether the numbers are from the latest run or from a tab you left open all evening.
Tomorrow versus the rest of the week
Tomorrow is the most stable day in the strip besides what is already happening today. Days further out swing more because small errors grow with time. If tomorrow changes by a little, do not be surprised when a day five steps away changes by more. Use the far days for a trend, such as a warmer stretch or a wetter pattern, and use tomorrow for the actual schedule.
An ordinary revision is not an emergency notice. This site shows the forecast it receives. It does not turn a higher rain chance into a warning. If you need watches, warnings, or evacuation instructions, use the government weather authority for your area.
When you come back in the morning, read tomorrow as its own block: high, low, condition, rain, and wind. Compare those items with the decision you already made. Ignore the rest unless one of them crossed a line that changes the plan.